1. Field of the Invention
The present invention relates to a source follower and a voltage follower, and more particularly to a semiconductor device having a source follower and a voltage follower formed by using a thin film transistor in a driver circuit.
2. Description of the Related Art
In an active matrix semiconductor display device, pixels having display elements are arranged in matrix in a pixel portion and each pixel is supplied with video signals for controlling the drive of the display elements through a plurality of signal lines provided in the pixel portion. These signal lines have load capacitance caused by display elements or other circuit elements in the pixel. Therefore, when the current is not supplied enough, the load capacitance is not charged rapidly and a video signal to be inputted to each pixel are largely delayed or do not rise nor fall sharply. In particular, a larger pixel portion in a display device tends to suffer from the aforementioned problem since load capacitance between the wirings are increased as wiring length is increased in accordance with the increased pixels.
In view of the aforementioned problem, such circuit as a source follower or a voltage follower for transforming an impedance are typically provided on the output side of a signal line driver circuit. The source follower in particular has a rather simple configuration that the drain of a transistor is fixed at a constant potential, the gate is used for input, and the source is connected to a constant current supply and used for output. By using these circuits, an area required for the signal line driver circuit does not have to be expanded much even when signal lines are increased in accordance with the increased resolution, therefore, they are used as a typical impedance transformer for a signal line driver circuit of a semiconductor display device. By providing an impedance transformer on the output side of the signal line driver circuit, current supply to the signal line can be increased and a delay or blunted rise and fall of a video signal can be avoided.
An active matrix semiconductor display device formed by using an inexpensive glass substrate can not easily be miniaturized since a periphery (frame area) of the pixel portion required for mounting occupies more area as the resolution becomes higher. Thus, a method for implementing an IC formed by using a single crystalline silicon wafer is considered to be nearing its mature phase, therefore, a technology for integrating a signal line driver circuit and a scan driver circuit on the same glass substrate as a pixel portion, that is referred to as System On Panel is now focused on.
A thin film transistor, however, has a large variation in characteristics as compared to a single crystalline MOS transistor. A variation in threshold voltage in particular is directly reflected to an output voltage of a source follower and a voltage follower. FIG. 9A shows a circuit diagram of a typical source follower. In the source follower shown in FIG. 9A, an input potential Vin is supplied to the gate (G) of a transistor 901 and a potential Vdd (Vdd>Gnd (potential of the ground)) is supplied to the drain (D) from a power supply. The source (S) is connected to a constant current supply 902 and the potential of the source corresponds to an output potential Vout.
The output potential Vout of the source follower having the aforementioned configuration can be obtained by the following formula shown in [Formula 1]. Note that Vgs corresponds to a voltage (gate voltage) that deducted the potential of the source from the potential of the gate.Vout=Vin−Vgs   [Formula 1]
The potential of this gate voltage Vgs is dependent on the relation between the gate voltage Vgs and a drain current Id. In the case where the transistor 901 is operated in a saturation region, the drain current Id can be obtained by the following formula shown in [Formula 2]. Note that i is mobility, Co is the gate capacitance per unit area, W/L is the proportion of the channel width W to the channel length L of a channel formation region, and Vth is a threshold voltage.Id=μCoW/L(Vgs−Vth)2/2   [Formula 2]
In Formula 2, μ, Co, W/L, and Vth are all fixed values determined by each transistor. The drain current Id of the transistor 901 is approximately determined by the constant current supply 902. Therefore, when the threshold voltage Vth is constant, it is found from Formula 2 that a predetermined gate voltage Vgs can be obtained. In other words, the gate voltage Vgs varies when the threshold voltage varies, which ends in the variation in the output potential Vout.
FIG. 9B shows measured values of the output potential Vout relatively to the input potential Vin of the source follower shown in FIG. 9A. As shown in FIG. 9B, the output potential Vout has a variation according to each source follower. This variation in the output of the source follower provided on the output side of the signal line driver circuit cause a variation in the potential of the video signal of each line, and visibly appear as a luminance variation in stripe shapes.